1. Field of the Invention:
The present invention relates to novel curable polyorganosiloxane compositions and, more especially, to such novel polyorganosiloxane compositions containing a catalytically effective amount of a tin monochelate crosslinking catalyst produced by reacting a tin bischelate with an organic carboxylic acid.
2. Description of the Prior Art:
Many tin compounds have heretofore been proposed to this art as a catalyst for crosslinking polyorganosiloxane compositions and, in particular, RTV compositions (room temperature vulcanizable compositions), whether in a single pack of in two packs, otherwise known as single- or two-component compositions.
The most widely used compounds are tin carboxylates such as tributyltin monooleate, tin 2-ethylhexanoate or dialkyltin dicarboxylates such as dibutyltin dilaurate and dibutyltin diacetate (see Noll, Chemistry and Technology of Silicones, page 337, Academic Press, 1968--2nd edition).
In U.S. Pat. No. 3,186,963, such a tin catalyst is described which is the reaction product of a dialkyldialkoxysilane with a tin carboxylate.
In Belgian Patent No. 842,305, the catalyst is the reaction product of an alkyl silicate or of an alkyltrialkoxysilane with dibutyltin diacetate.
And in U.S. Pat. No. 3,708,467, a catalyst system is described which is a mixture of certain tin salts with a specific titanium chelate, in a single-component composition.
Lastly, in U.S. Pat. Nos. 4,517,337 and 4,554,310 the use of diorganotin bis(.beta.-diketone) is described for the crosslinking of neutral single-component compositions (U.S. Pat. Nos. 4,517,337 and 4,554,310) or for single- and two-component compositions (EP-A-147,323).
Although EP-A-147,323 represents a significant advance in the quest for a tin catalyst useful for both single- and two-component compositions, it has become apparent that diorganotin bis(.beta.-diketonates) exhibit a core setting time which is a little too slow, particularly in the case of the two-component compositions.
The problem which typically arises in the case of the single-component compositions is essentially that of storage stability and of the retention of physicochemical properties (extrudability, pourability, setting time) of the composition and maintaining these properties by the reticulate (mechanical properties, hardness, elongation, tear strength, adhesiveness, and the like).
Thus, need exists in this art for a catalyst which crosslinks very rapidly on exposure to atmospheric moisture, not only on the surface thereof, but which at the same time provides a thorough crosslinking uniformly therethrough which is as complete as possible, and which is also active in minor amounts, while reducing to the minimum the reticulate degradation reactions which are inherent in the presence of tin.
With regard to the three-dimensional shaped article or reticulate thus obtained, the same problems as exist in the case of the single-component compositions also exist in the case of the two-component compositions, but, in addition, the exposure or open time (that is to say, the time during which the composition may be employed after mixing without hardening) must be sufficiently long to permit its effective use, but sufficiently short to produce a molded object capable of being handled not later than 24 hours after the production thereof.
This catalyst must therefore provide a good compromise between the open time of the catalyzed mixture and the time after which the molded object can be handled. In addition, the catalyst must impart to the catalyzed mixture a spreading time which does not vary as a function of the storage period.